is (8) Kt-d5, attacking the Knight f6 for the second time.
Although the Knight is twice protected, Black will naturally
endeavor to prevent an accumulation of hostile pieces on the
point f6, and he can do that indeed very easily by playing (7)
...., B-e6 with a view toward capturing the Knight if he should
move to d5.

The exchange (8) Kt-d5, Bxd5; (9) Bxd5 is manifestly not
desirable for White, as it does not improve in any way the
mobility of his men. On the contrary, White loses a move, as the
result of the maneuver is only a change of location of the King's
Bishop who was already developed while the principle of speedy
development demands that no piece should be moved twice in the
opening until ALL pieces are developed.

Unless Black's move involves a threat which needs immediate
attention White should consider only such moves as will secure a
possibility of development for his Rooks, that is maneuvers which
are liable to produce an open file somewhere on the board.

+---------------------------------------+
8 | #R | | | #Q | | #R | #K | |
|---------------------------------------|
7 | #P | #P | #P | | | #P | #P | #P |
|---------------------------------------|
6 | | | #Kt| #P | #B | #Kt| | |
|---------------------------------------|
5 | | | #B | | #P | | ^B | |
|---------------------------------------|
4 | | | ^B | | ^P | | | |